1. Field of the Invention
This invention relates to a photosensitive composition containing a novel compound capable of forming a free radical upon exposure by light and, more particularly, the invention relates to a photosensitive composition containing a 2-trihalomethyl-5-aryl-1,3,4-oxadiazole compound.
2. Description of the Prior Art
Compounds which are decomposed upon exposure to light to form free radicals (hereinafter referred to as free radical generating agents) are well known in the graphic arts. These compounds are widely used as photopolymerization initiators for photopolymerizable compositions, photoactivators for free radical photographic compositions and photoinitiators for a reaction catalyzed by the acid formed by the action of light. Using such free radical generating agents various photosensitive materials useful for printing, reproduction, duplication, and other image forming systems are prepared.
An organic halogen compound provides a halogen free radical such as a chlorine free radical and a bromine free radical upon photodecomposition. These halogen free radicals are good hydrogen abstractors and produce acids when hydrogen donors are present. The applications of these compounds to photopolymerization and free radical photography are described in J. Kosar Light-Sensitive Systems, pages 180-181 and 361-370, published by J. Wiley & Sons, New York, 1965.
Typical examples of the compound forming a halogen free radical by the action of light are carbon tetrabromide, iodoform, tribromoacetophenone, etc., and they have been used widely. However, these free radical generating agents tend to sublime and have a bad odor and hence when these compounds are used for photosensitive materials having photosensitive layers formed on supports, they may be lost during production, use, and storage of the photosensitive materials as well present hygienic troubles. Also, these free radical generating agents have troubles on compatibility with other elements contained in the photosensitive layers. Furthermore, because these free radical generating agents have a low sensitivity (with respect to photodecomposition) to light sources usually used in case of producing, for example, printing plates (e.g., a metal halide lamp, etc.), a large amount of the compounds must be added to obtain sufficient effects. However, if a large amount of these compounds is present in photosensitive layers, they usually exert a very bad influence upon the mechanical property and developing property of the photosensitive layers.
Wang et al., U.S. Pat. No. 4,040,992 discloses the use of compounds of the following formula as photopolymerizable initiator: ##STR2## wherein A represents an oxygen atom, a sulfur atom, or --NR-- (wherein R is a hydrogen atom or an alkyl group); Z represents --CH.sub.2 X, --CHX.sub.2 or --CX.sub.3 (wherein X is a halogen atom); E represents a hydrogen atom, a halogen atom, an alkyl group, an alkoxy group, an aryl group, an aryloxy group, a hydroxy group, an amino group, a carbonylalkoxy group, or a cyano group; and D represents an alkyl group, a substituted alkyl group, an aryl group, or a substituted aryl group.
However, even though these compounds have low volatility and have good compatibility with other various components used in photosensitive compositions, these compounds do not provide sufficient increase in sensitivity which would be practically useful when they are incorporated in photopolymerizable compositions or printing-out compositions.